1. Field of the Invention
Embodiments of the invention relate to methods and apparatuses for generation of high-quality high pressure steam from produced water in the heavy oil production industry.
2. Description of the Related Art
Heavy oil recovery processes employ the “steamflood” technique, in which steam is injected to the oil bearing formations to improve the recovery of heavy oils. Typically, several units of steam are required for recovering each unit of oil. Heavy oil is a viscous fluid, and the heat from steam reduces the viscosity of the oil allowing it to flow to a production well. In the process of heating the oil, steam condenses. This condensed steam is called produced water. The mixture of oil and produced water is pumped to the surface through the production well. The mixture of oil and produced water is separated in individual fractions in the conventional de-oiling processes typical in oil recovery operations.
Due to the large amount of steam that is normally required for heavy oil recovery, it is desirable to recycle the produced water for reuse. This requires treatment of the produced water to make it suitable as feedwater to a steam generator or boiler. Typically a single evaporator and mechanical vapor compressor are used to treat produced water for production of distillate as boiler feedwater.
Unfortunately, the current approaches to produced water purification and distillate production have drawbacks and disadvantages that hinder achievement of desired distillate purity in a cost-effective manner. The current practice of disposing of wastewater by injecting in a deep well formation in heavy oil facilities is also not a very environmentally prudent approach. Therefore, there is a need for a more cost-effective system for treating produced water that can produce a relatively pure feedwater stream for a steam generation system and can offer an environmentally friendly approach to waste disposal at the heavy oil facilities.
Evaporation technology has been applied to the heavy oil sands process to generate distillate that is suitable for steam generation. For example, steam is used in the enhanced oil recovery plants through a steam injection process that liquefies heavy oil and transport the heavy oil to the surface where it can be separated from water and further refined. The produced water that is separated from the oil is processed by the evaporator. This processes the water into distillate suitable for steam generation. The steam generators can be either once through steam generators (OTSGs) or high pressure drum-type boilers.
Conventional Mechanical Vapor Compression (MVC) evaporation technology uses vertical falling film evaporators to vaporize produced water. The droplets that are entrained in this water vapor tend to remain entrained and mix with the condensed vapor to contaminate the distillate quality. Typically these droplets are removed with a demister located in the annular area of the evaporator sump. The internal demister offers only one stage of droplet elimination; therefore, the process is limited in the distillate purity that may be achieved. This underachievement limits the use of this distillate. Drum boilers must be designed and operated at lower pressures because of this deficiency. OTSGs are also limited in the vaporization rate they can achieve.
Further problems ensue for the conventional purification technology because of the quiescent volume in the evaporator sump. A conventional evaporator has a diameter significantly larger than the heat exchanger tube bundle. As a result, when the falling film falls into the sump it primarily moves directly downward with relatively little agitation of the annular volume of the cylindrical sump. The lack of mixing in this region allows components such as oil and grease to accumulate in this quiescent volume. There are several drawbacks inherent in this design including a) the accumulation of oil and grease in the evaporator system during normal operation, b) the need for an additional oil removal step dedicated to the accumulated oil in the wide-sump evaporator, c) higher concentrations of oil and grease in the evaporator cause greater concentrations of oil and grease in the purified distillate which pollutes the primary product.
Another significant drawback of the conventional treatment approach is that the evaporator system operates at a thermal efficiency less than ideal. This requires excess energy to be removed from the system. The conventional approach achieves energy balance by venting steam from the evaporator system. Since this vent steam typically contains concentrations of hydrogen sulfide and other environmentally harmful substances, the vent steam is a waste stream that is not suitable for vent to the atmosphere. Instead it must be collected and disposed in a flare, catalytic oxidizer or other disposal system. This limitation represents a significant drawback because it increases the capital cost and operating costs of the disposal system.